Products having high wear-resistance are extensively used and there is a constant need for less expensive products having the same or better performance as/than existing products. Only valve seats inserts are produced in an amount of more than 1,000,000,000 components annually.
The manufacture of products having high wear-resistance may be based on e.g. powders, such as iron or iron-based powders, including carbon in the form of carbides.
Carbides are very hard and have high melting points, characteristics which give them a high wear resistance in many applications. This wear resistance often makes carbides desirable as components in steels, e.g. high speed steels (HSS), that require a high wear resistance, such as steels for drills, lathes, valve seat inserts and the likes.
A VSI in a combustion engine is a ring that is inserted where the valve comes in contact with the cylinder head during operation. The VSI is used to limit the wear, caused by the valve, on the cylinder head. This is done by using a material in the VSI that can resist wear better than the cylinder head material, without wearing on the valve. The materials used for VSI are cast materials or more commonly pressed and sintered PM materials.
Producing a valve seat insert with powder metallurgy offers a wide flexibility in composition of the VSI and a very cost effective product. The method of fabricating a PM valve seat insert starts with preparation of a mix which includes all ingredients needed in the final component. The powder mix most commonly includes an iron or low alloyed powder serving as matrix in the final component, elemental alloying elements such as C, Cu, Ni, Co etc which should to a lower or higher extent diffuse into the matrix material and enhance strength and hardness. Further hard phase materials containing carbides and similar phases can be added to increase the wear resistance of the alloy. It is also common to have machinability enhancers added to decrease tool wear when machining the finished product, as well as solid lubricants in order to assist the lubrication during service in the engine. Further, in all press ready mixes evaporative lubricants are added to assist compaction and ejection of the compacted component. A known VSI material, produced by Powder Metallurgy, is based on high speed steel powder as carbide containing matrix material. All powders used normally have a particle size of less than 180 μm. The average particle size of the mix is usually between 50 to 100 μm to allow the mix to flow and facilitate production. The alloying and lubricant additives are in many cases finer in particle size compared to the matrix powder to improve distribution of alloying elements in the powder mix and finished component.
The powder mix is then fed into a tool cavity with the shape of a VSI ring. An axial pressure between 400-900 MPa is applied resulting in a near net shape metallic VSI component having a density between 6.4-7.3 g/cm3. In some instances dual compaction is used to decrease the use of expensive alloying elements. In dual compaction two different powder mixes are used. One more expensive with excellent wear properties creating the wear surface of VSI facing the valve and one less costly to give the desired height of the component. After the compaction the individual grains are only loosely bonded through cold welding, and a subsequent sintering operation is required to allow the individual particles to diffuse together and to distribute alloying elements. Sintering is usually performed at temperatures between 1120° C. and 1150° C. but temperatures up to 1300° C. can be used, in a reducing atmosphere usually based on Nitrogen and Hydrogen. During sintering or after, copper can be infiltrated in the pores of the component to increase hardness and strength as well as improve heat conductivity and wear properties. In many cases subsequent heat treatments are performed to reach final properties. In order to achieve desired geometrical accuracy of the VSI it is machined to desired size. The final machining is in many cases done after VSI is mounted in the cylinder head. The final machining is done in order to give the VSI and inverted valve profile and to have small dimensional variations.
Examples of conventional iron-based powders with high wear resistance are disclosed in e.g. the U.S. Pat. No. 6,679,932, relating to a powder mixture including a tool steel powder with finely dispersed carbides, and the U.S. Pat. No. 5,856,625 relating to a stainless steel powder.
W, V, Mo, Ti and Nb are strong carbide forming elements which make these elements especially interesting for the production of wear resistant products. Cr is another carbide forming element. Most of these conventional carbide forming metals are, however, expensive and result in an inconveniently high priced product. Thus, there is a need within the powder metallurgical industry for a less expensive iron-based powder, or high speed steel, which is sufficiently wear resistant for applications such as for valve seats or the like.
As chromium is a much cheaper and more readily available carbide forming metal than other such metals used in conventional powders and hard phases with high wear resistance, it would be desirable to be able to use chromium as principal carbide forming metal. In that way the powder, and thus the compacted product, can be more inexpensively produced.
The carbides of regular high speed steels are usually quite small, but in accordance with the present invention it has now unexpectedly been shown that powders having equally advantageous wear resistance, for e.g. valve seat applications, may be obtained with chromium as the principal carbide forming metal, provided that a sufficient amount of large carbides exists, supported by a minor amount of finer and harder carbides.